where \(G_k\)  is the momentum space Green's function of the operator \(D\) . One can perturbatively calculate \(v_F(\alpha )\)  for \(\alpha <1\)  and up to first order in perturbation, one finds that \(v_F ( \alpha = 1/\sqrt{3}) =0\) . This ensures flattening of the erstwhile Dirac cone at the point \( \mathbf {k} =0\) , ie the \(K\)  point. For a value of \(w_1\)  phenomenologically collected, this happens for the angle \(\theta \approx 1.05^{ \circ } \)  [1]}, which is famously known as the `magic angle'.
